1. Field of the Invention
The present invention relates broadly to the processing of information obtained by a sonic borehole tool. More particularly, the invention relates to the downhole processing of compressional sonic wave data via the use of data compression. The invention finds particular use in both logging while drilling applications where the transmission rate of data is limited due to the absence of a telemetry cable, as well as to array sonic tools applications where the data transmission through the telemetry cable is a constraint on the logging rate.
2. State of the Art
Sonic logging is a well developed art, and details relating to sonic logging tools and techniques are set forth in "Porosity Logs"; Schlumberger Log Interpretation Principles/Applications, Chapter 5, Schlumberger Educational Services, Texas (1987); A. Kurkjian, et al., "Slowness Estimation from Sonic Logging Waveforms", Geoexploration, Vol. 277, pp. 215-256 (1991); C. F. Morris et al., "A New Sonic Array Tool for Full Waveform Logging," SPE-13285, Society of Petroleum Engineers (1984); A. R. Harrison et al., "Acquisition and Analysis of Sonic Waveforms From a Borehole Monopole and Dipole Source . . . " SPE 20557, pp. 267-282 (September 1990); C. V. Kimball and T. L. Marzetta, "Semblance Processing of Borehole Acoustic Array Data", Geophysics, Vol. 49, pp. 274-281 (March 1984); U.S. Pat. No. 4,131,875 to Ingram; and U.S. Pat. No. 4,594,691 to Kimball et al., all of which are hereby incorporated by reference herein in their entireties. A sonic logging tool typically includes a sonic source (transmitter), and a plurality of receivers which are spaced apart by several inches or feet. In the borehole arts, a sonic signal is transmitted from a sonic source and received at the receivers of the borehole tool which are spaced apart from the sonic source, and measurements are made every few inches as the tool is drawn up the borehole. The sonic signal from the transmitter or source enters the formation adjacent the borehole, and the arrival times and perhaps other characteristics of the receiver responses are recorded. Typically, compressional (P-wave), shear (S-wave) and Stoneley arrivals and waves are detected by the receivers and are processed. The processing of the data is often accomplished uphole. Regardless, the information which is recorded is typically used to find formation parameters such as formation slowness (the inverse of sonic speed) and semblance, from which pore pressure, porosity, and other determinations can be made. In certain tools such as the DSI (Dipole Sonic Imager) tool (a trademark of Schlumberger), the sonic signals may even be used to image the formation.
Even though some of the borehole tools of the art can transfer the data uphole via a telemetry cable, because the communications rates are limited, either the rate of movement of the borehole tool through the borehole must be limited, or data must be stored by the borehole tool. Typically, the logging rate is chosen so that all information obtained downhole may be sent via the telemetry cable. It is possible, however, to provide the borehole tool with a memory for storing large amounts of data that are collected downhole. Because it is not feasible to provide the borehole tool with enough memory to store all of the information that could be obtained during a trip through the borehole, the borehole tool might have to be tripped out of the well so that the memory can be downloaded. The borehole tool would then placed back into the borehole so that additional information could be obtained.
Many different techniques for processing the sonic wave signals are known in the art in order to obtain information regarding the borehole and/or formation. Typically, the processing involves digitizing the received signal at a desired sampling rate and then processing the digitized samples according to desired techniques. Examples may be found in U.S. Pat. No. 4,594,691 to Kimball et al. and the references cited therein, as well as in articles such as A. R. Harrison et al., "Acquisition and Analysis of Sonic Waveforms From a Borehole Monopole and Dipole Source . . . " SPE 20557, pp. 267-282 (September 1990).
Recently, compressional slowness has been computed using Slowness-Time Coherence (STC) processing. C. V. Kimball and T. L. Marzetta, "Semblance Processing of Borehole Acoustic Array Data", Geophysics, Vol. 49, pp. 274-281 (March 1984). In STC processing, the measured signal is time window "filtered" and stacked, and a semblance function is computed. The semblance function relates the presence or absence of an arrival with a particular slowness and particular arrival time. If the assumed slowness and arrival time do not coincide with that of the measured arrival, the semblance takes on a smaller value. Consequently arrivals in the received waveforms manifest themselves as local peaks in a plot of semblance versus slowness and arrival time. These peaks are typically found in a peak-finding routine discussed in the aforementioned article by Kimball and Marzetta.
While not yet available, sonic tools have been proposed for the logging or measuring while drilling (LWD) arts. Sonic LWD will have many potential applications in oil field services including seismic correlation while drilling, pore pressure and porosity determinations, and mechanical property determinations. Because no telemetry cables will be available in these proposed arts, the transfer of data will have to be accomplished via the use of pulses in the flow of the drilling mud (i.e., mud pulse telemetry). With such a limited data transfer mechanism, even with the best data transmission schemes such as are used in the PowerPulse tool produced by (and a trademark of) Anadrill of Sugar Land, Tex., data can only be transferred at a rate of 32 Hz or less. While the drilling penetration rate is very slow relative to normal logging rates, data acquisition still can far exceed the highest data transmission rates in the mud. Thus, in any proposed sonic LWD art, processing of data downhole will be highly desirable, even though the downhole computing power will be limited. Otherwise, much data will have to be stored in the memory of the downhole tool, and frequent trips out will be required.